This invention relates to ceramic ignitors for gas and liquid fuel burning devices such as furnaces, water heaters, clothes dryers, kitchen stoves, and ranges. Such ignitors are becoming increasingly more important as the world's fuel supplies diminish and improved energy conservation methods are sought. Ignitors of this type are used to replace pilot lights on fuel burning devices, which often account for as much as 30-50% of the total gas consumed. The general concept of electrically operated ignitors is not new, ceramic resistance ignitors having been first used prior to World War II. Early versions of such ignitors were exclusively prepared from dense sintered silicon carbide impregnated with silicon.
However, silicon carbide has a poor resistance to spark erosion at elevated temperatures. Various additions have been made to the silicon carbide by those skilled in the art in attempts to improve these spark erosion characteristics. Other materials such as molybdenum disilicide and tungsten disilicide, have also been used in ignitor applications.
Ignitors designed for use in household applications must have a very short response time. As used herein the term "response time" refers to the time between initial application of current to the ignitor, and the maximum temperature output of the ignitor.
Electrical consumption can be lowered significantly by using a higher resistant electrical conductor, but such a conductor must be capable of achieving the desired temperatures rapidly at line voltage (120 volts) to provide an efficient and economical ignitor for household appliance applications.
In addition, such an ignitor must have a high resistance to thermal shock, a high degree of strength, and must be capable of prolonged service life over more than about 200,000 thermal cycles. In most natural gas household applications an ignitor should be capable of heating from room temperature to about 2600.degree. F. in less than 30 seconds. These properties must be maintained over the full service life of the ignitor, if the device is to be economically useful.
Preferably, the resistance-temperature characteristics of such an ignitor should show a low resistivity at room temperature in order to promote the fast start-up necessary for a heat-up response time of less than 30 seconds. In the high temperature range (about 2600.degree. F.) the negative slope of the electrical resistivity-temperature curve should level off to avoid any overheating of the ignitor device.
In addition to the above characteristics, an ignitor designed for economical household use should not require more than about 100-110 watts of power in normal operation. If the desired response time can be achieved in this range of power demand, the use of electrical ignitors as replacements for pilot lights will result in significant overall energy savings.
Also, the structure of a ceramic ignitor should be such as to avoid any change in resistivity at the connection between the ceramic material and the electrical lead wires in order to increase the service life of the connection. Excessive changes in resistance at this connection point could lead to overheating and a corresponding deterioration of the ceramic-metal joint.
The wired parts of the ignitor should also be protected from the possibility of direct contact with any burner flame, but the heating element must be adequately exposed to the fuel medium for rapid ignition.
Ignitors in the prior art formed from recrystallized silicon carbide have a relatively low resistivity, and therefore require an electric power demand of up to 500 watts to reach temperatures required in fuel ignitor applications. In this situation, the amount of fuel energy savings is wasted due to excessive use of electrical energy.
In addition to their high power demand, discussed previously, the ignitors of the prior art have been unable to achieve heat-up times of less than 30 seconds at line voltage. In general, prior art ignitors respond in 30-60 seconds at line voltage.
It has been discovered that a ceramic ignitor having the requisite strength, temperature, service life, resistance, and voltage characteristics in combination with greatly improved response time, can be constructed from pure lanthanum chromite or from lanthanum chromite which has been doped with specific percentages of magnesium, niobium, or strontium ions alone or in combination.
Particularly, it has been found that an ignitor formed from these materials is capable of efficient operation at line voltage with significantly lower power demand than ignitors of the prior art.
Lanthanum chromite has been used in the prior art in combination with other materials for the manufacture of electrodes and semiconductors as disclosed, for example, in Aubin et al, U.S. Pat. No. 3,730,911. However, lanthanum chromite has not been previously suggested or used for ceramic ignitor construction.
It is also known in the art to modify the electronic properties of lanthanum chromite by doping it with certain metallic ions which substitute for the lanthanum or chromium ions at the A- or B- sites of the Perovskite lattice. Staut et al, U.S. Pat. No. 3,974,108 discloses a method for such doping using strontium, calcium or magnesium ions.
As with pure lanthanum chromite, doped lanthanum chromite has not been suggested or used previously for ceramic ignitor construction.
Accordingly, it is a primary object of the invention to provide improved materials for the formation of ceramic ignitors.
It is a further object of the invention to provide an improved ceramic ignitor which is capable of more rapid heat-up response.
A still further object of the invention is to provide an improved ceramic ignitor which is capable of more efficient operation at line voltage.
Yet another object of the invention is to provide a new and improved ceramic ignitor having a high resistance to thermal shock, a high strength, a prolonged service life, and a faster response time.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.